An ignition plug for use in combustion apparatus such as an internal combustion engine includes, for example, a center electrode extending in the direction of an axial line, an insulator provided externally of the outer circumference of the center electrode, and a cylindrical metallic shell attached externally to the insulator. Also, a ground electrode is joined to a forward end portion of the metallic shell, and a gap (spark discharge gap) is formed between the center electrode and the ground electrode for generating spark discharge. Additionally, the metallic shell has, on its inner circumferential surface, an elongated protrusion protruding radially inward and adapted to allow an outer circumferential surface of the insulator to be seated thereon and has, on its outer circumferential surface, a threaded portion to be threadingly engaged with a mounting hole of the combustion apparatus.
Meanwhile, the metallic shell is formed generally through extrusion and cutting work. Specifically, a columnar metallic shell intermediate formed of a predetermined metal material is placed in a tubular die; then, the metallic shell intermediate is deformed under pressure at its forward and rearward sides by means of predetermined jigs so as to form holes at the forward and rearward sides, respectively, of the metallic shell intermediate. Then, by use of a plurality of jigs, the formed holes are deformed under pressure so as to increase stepwise in depth and diameter; finally, the opposite holes of the metallic shell intermediate are connected so as to communicate with each other. At this time, an annular protrusion which is to become the elongated protrusion is formed on the inner circumferential surface of the metallic shell intermediate. Next, cutting work, etc., are performed on, for example, that portion of the internal circumferential surface of the metallic shell intermediate which is located forward of the protrusion, so as to adjust the shape of the metallic shell intermediate, thereby yielding a metallic shell tubular intermediate. Finally, rolling is performed on an outer circumferential surface of the metallic shell tubular intermediate so as to form the threaded portion, thereby yielding the metallic shell (refer to, for example, Japanese Patent No. 4210611).